Field of the Invention
The invention concerns a method for quality control in planning radiotherapy of a patient, as well as a radiotherapy planning computer, a magnetic resonance apparatus and a non-transitory, computer-readable data storage medium for implementing such a method.
Description of the Prior Art
With radiation therapy, target tissue, such as a tumor, of a patient is irradiated with ionizing radiation. External radiation therapy, in which radiotherapy of the body of the patient is implemented from a radiation source outside of the body, is known for use in this context. Internal radiation therapy, also called brachytherapy, is also known. With brachytherapy, radiation sources, composed of radioactive substances, are introduced into the body of the patient to locally damage or destroy target tissue in the body of the patient.
It is known to plan and/or monitor radiation therapy of a patient by the use of imaging. For this purpose a radiotherapy plan is conventionally created with the use of medical image data of the patient, which have been created using a three-dimensional imaging method. Computed tomography image data (CT image data) is conventionally used for this purpose. Using the CT image data the target volume for radiotherapy can be defined, and surrounding tissue that is to be spared—for example neuronal tissue—can be localized. Furthermore, the intensity values of the image voxels of the image data (measured in “Hounsfield Units”) map the electron density at the corresponding location in the body of the patient to a good approximation, since the intensity values of the image voxels are based on absorption of the imaging X-ray radiation at the associated locations. In this way the CT image data can be converted particularly easily into an electron density map for radiotherapy planning. In the case of radiotherapy, the intensity of the interaction of the radiotherapy radiation correlates with the electron density in the body, so the attenuation of the radiation as it passes through the body can be calculated from the CT image data relatively easily. Due to this property, the preference has conventionally been to use CT image data when preparing radiotherapy planning.
There is a need, however, to use other imaging modalities in radiotherapy planning that have better soft tissue contrast in order to enable improved identification of target organs and/or at-risk organs. One imaging modality that satisfies the need for better soft tissue contrast is magnetic resonance imaging (MR imaging) by the use of a magnetic resonance apparatus (scanner). With MR imaging, the contrast depends on the distribution of the spin density of nuclear spins that have been excited, the interaction of the spins among each other and/or with their surroundings. A soft tissue contrast can be attained that is far superior to the contrast that can be attained with computed tomography.
In a magnetic resonance apparatus, also called a magnetic resonance tomography system, the body of an examination person, in particular a patient, to be examined is conventionally exposed to a relatively high basic magnetic field, for example of 1.5 or 3 or 7 tesla, produced by a basic field magnet. In addition, gradient pulses are activated by a gradient coil arrangement. Radio-frequency pulses (excitation pulses) are emitted by a radio-frequency antenna unit by suitable antenna devices, and this leads to the nuclear spins of specific atoms, excited in a resonant manner by these radio-frequency pulses, being tilted by a defined flip angle with respect to the magnetic field lines of the basic magnetic field. When the nuclear spins relax, radio-frequency signals, known as magnetic resonance signals, are emitted which are received by suitable radio-frequency antennae and then processed further. Finally, the desired image data are reconstructed from the raw data acquired in this way.
Combined use of CT imaging and magnetic resonance-imaging is known for radiotherapy planning (planning radiotherapy). The acquired CT image data and magnetic resonance image data are then typically overlaid by image registration for radiotherapy planning. The main benefit of CT image data in radiotherapy planning is to supply electron densities and geometric precision, while the magnetic resonance image data typically supplies better clinical information on target organs and/or at-risk organs.
One development in recent years under the designation of magnetic resonance-only radiotherapy planning (“MR-only RT Planning”, MRORTP) envisages eliminating the CT image data from the planning process in suitable clinical applications. In this way, radiotherapy planning should occur using only magnetic resonance image data acquired from the patient. In this way, for example, a number of necessary patient scans (only magnetic resonance scans instead of both CT scans and magnetic resonance scans) can be reduced and/or possible registration errors between the CT image data and magnetic resonance image data can be avoided.
MR-only RT planning, however, poses new challenges. The electron density map required for the dosage calculation in the radiotherapy planning can be determined from magnetic resonance image data only with greater algorithmic effort than is the case for CT data. In contrast to CT image data, image contrasts in magnetic resonance image data typically do not have a clear physical relationship with the electron density and therewith photon attenuation. For example, neither bone regions nor air regions exhibit any signal in conventional magnetic resonance contrasts. This means bone regions as well as air regions are typically both black in the magnetic resonance image data, even though they have different electron densities and therefore different photon attenuations.